In a certain particle accelerator, protons travel around a circular path of diameter 18.0 m in an evacuated chamber, whose residual gas is at 290 K and 1.56 x 10-6 torr pressure. (a) Calculate the number of gas molecules per cubic centimeter at this pressure. (b) What is the mean free path of the gas molecules if the molecular diameter is 2.12 x 10-8 cm?